FIG. 1 shows a high-bypass gas turbine engine in the form of a turbofan 10. The engine 10 comprises, in axial flow series, an air intake duct 11, an intake fan 12, a bypass duct 13, an intermediate pressure compressor 14, a high pressure compressor 16, a combustor 18, a high pressure turbine 20, an intermediate pressure turbine 22, a low pressure turbine 24 and an exhaust nozzle 25. The fan 12, compressors 14, 16 and turbines 20, 22, 24 all rotate about the major axis of the gas turbine engine 10 and so define the axial direction of gas turbine engine. Alternatively, the shaft power could be used to drive other mechanical loads, such as a marine propeller or an electrical generator.
Air is drawn through the air intake duct 11 by the intake fan 12 where it is accelerated. A significant portion of the airflow is discharged through the bypass duct 13 generating a corresponding portion of the engine 10 thrust. The remainder is drawn through the intermediate pressure compressor 14 into what is termed the core of the engine 10 where the air is compressed. A further stage of compression takes place in the high pressure compressor 16 before the air is mixed with fuel and burned in the combustor 18. The resulting hot working fluid is discharged through the high pressure turbine 20, the intermediate pressure turbine 22 and the low pressure turbine 24 in series where work is extracted from the working fluid. The work extracted drives the intake fan 12 (or other load), the intermediate pressure compressor 14 and the high pressure compressor 16 via shafts 26, 28, 30. The working fluid, which has reduced in pressure and temperature, is then expelled through the exhaust nozzle 25 and generates the remaining portion of the engine 10 thrust.
FIG. 2 shows an external view of part of a conventional combustor 18 of the gas turbine engine 10. The combustor comprises a combustor liner which resides within a combustor casing 36. The combustor liner defines an annular space in which fuel is combusted. Fuel is provided to the annular space by a plurality of fuel injectors 34 located at several annular locations around the casing 36, which are in turn provided with fuel by a fuel conduit 32. The conduit 32 is connected to the injectors 34 by connectors 38. The conduit comprises a metal alloy (steel or Inconel™ for example), which provides the necessary high strength and high temperature capability to perform in the high temperature environment in which the conduit 32 operated. The conduit 32 has multiple bends and curves, as can clearly be seen in FIG. 3. These bends are necessary in order to accommodate thermal expansion of the conduit 32 without overstressing the material, which may have a strain limit of approximately 0.2 to 0.3%. these bends also provide resistance to damage from vibration, and are termed “pigtails” in the art. Bellows or flexible joints may be provided within the conduit system in order to provide further flexibility and resistance to internal pressure variations and vibration. Other types of combustors are also known, such as “staged combustors”. In any case, fuel must be delivered to fuel injectors within the combustor.
Other conduits in gas turbine engines (such as hydraulic, oil and air conduits) also have to survive similar or higher temperature conditions, and are subject to large stresses, thermal gradients and vibrations in use.
In some cases, flexible tubing made of plastics material is used instead of metal alloys. Examples include PTFE. Such tubing still generally requires pig tails to damp vibration.
However, the addition of extra material to provide bends in conduits increases the weight of the conduits. Existing conduits are also prone to failure, due in part to fretting from vibration. The necessary joints, bellows and different material interfaces provide potential failure points. The unit cost of these complex arrangements is also relatively high.
The present invention describes a fluid conduit for a gas turbine engine, a gas turbine engine comprising a fluid conduit, and a method of designing a fluid conduit which seeks to overcome some or all of the above problems.